Thermo-compression bonded electrical interconnect structure and method

ABSTRACT

An electrical structure and method for forming. The electrical structure includes a first substrate comprising a first electrically conductive pad, a second substrate comprising a second electrically conductive pad, and an interconnect structure electrically and mechanically connecting the first electrically conductive pad to the second electrically conductive pad. The interconnect structure comprises a non-solder metallic core structure and a first solder structure. The first solder structure electrically and mechanically connects a first portion of the non-solder metallic core structure to the first electrically conductive pad. A second portion of the non-solder metallic core structure is thermo-compression bonded to the second electrically conductive pad.

This Application is related to application Ser. No. 11/855,236 filed on Sep. 14, 2007.

FIELD OF THE INVENTION

The present invention relates to a thermo-compression bonded electrical interconnect structure and method for forming.

BACKGROUND OF THE INVENTION

Connections between structures are typically unreliable and subject to failure. Accordingly, there exists a need in the art to overcome at least one of the deficiencies and limitations described herein above.

SUMMARY OF THE INVENTION

The present invention provides an electrical structure comprising:

a first substrate comprising a first electrically conductive pad;

a second substrate comprising a second electrically conductive pad; and

an interconnect structure electrically and mechanically connecting said first electrically conductive pad to said second electrically conductive pad, wherein said interconnect structure comprises a non-solder metallic core structure and a first solder structure in direct mechanical contact with a first portion of said non-solder metallic core structure, wherein said first solder structure electrically and mechanically connects said first portion of said non-solder metallic core structure to said first electrically conductive pad, and wherein a second portion of said non-solder metallic core structure is thermo-compression bonded to said second electrically conductive pad.

The present invention provides an electrical structure comprising:

a first substrate comprising a first electrically conductive pad;

a second substrate comprising a second electrically conductive pad; and

an interconnect structure electrically and mechanically connecting said first electrically conductive pad to said second electrically conductive pad, wherein said interconnect structure comprises a non-solder metallic core structure, wherein a first portion of said non-solder metallic core structure is thermo-compression bonded to said first electrically conductive pad, and wherein a second portion of said non-solder metallic core structure is thermo-compression bonded to said second electrically conductive pad.

The present invention advantageously provides a simple structure and associated method for forming connections between structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross sectional view of an electrical structure, in accordance with embodiments of the present invention.

FIG. 2 depicts a first alternative to FIG. 1, in accordance with embodiments of the present invention.

FIG. 3 depicts a first alternative to FIG. 2, in accordance with embodiments of the present invention.

FIG. 4 depicts a first alternative to FIG. 3, in accordance with embodiments of the present invention.

FIG. 5A depicts a first alternative to FIG. 4, in accordance with embodiments of the present invention.

FIG. 5B depicts a first alternative to FIG. 5A, in accordance with embodiments of the present invention.

FIG. 6A depicts a second alternative to FIG. 5A, in accordance with embodiments of the present invention.

FIG. 6B depicts a first alternative to FIG. 6A, in accordance with embodiments of the present invention

FIGS. 7A-7H illustrate a process for generating the electrical structure 2C of FIG. 3 and/or the electrical structure of FIG. 4, in accordance with embodiments of the present invention.

FIGS. 8A-8E illustrate a process for generating the electrical structure of FIG. 1 and/or the electrical structure of FIG. 2, in accordance with embodiments of the present invention.

FIGS. 9A-9F illustrate a process for generating the electrical structure of FIGS. 5A-6B, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a cross sectional view of an electrical structure 2 a, in accordance with embodiments of the present invention. Electrical structure 2 a comprises a substrate 1, a substrate 4, a plurality of interconnect structures 5, an optional layer of adhesive 22, and an optional layer(s) 16 of underfill encapsulant material. Substrate 1 comprises a plurality of electrically conductive pads 10. Each pad of electrically conductive pads 10 may be connected to wires or electrical components within substrate 1. Substrate 4 comprises a plurality of electrically conductive pads 12. Each pad of electrically conductive pads 12 may be connected to wires or electrical components within substrate 4. Substrate 1 may comprise, inter alia, a semiconductor device (e.g., an integrated circuit chip, a semiconductor wafer, etc), a chip carrier (organic or inorganic), a printed circuit board, etc. Substrate 4 may comprise, inter alia, a semiconductor device (e.g., an integrated circuit chip, a semiconductor wafer, etc), a chip carrier (organic or inorganic), a printed circuit board, etc. Each interconnect structure 5 comprises a non-solder metallic (i.e., does not comprise any solder material) core structure 5 and a solder structure 14. Solder structure 14 comprises solder. Solder is defined herein as a metal alloy comprising a low melting point (i.e., about 100 degrees Celsius to about 340 degrees Celsius) that is used to join metallic surfaces together without melting the metallic surfaces. Each solder structure 14 comprises a portion of solder electrically and mechanically connecting a bottom side 19 of non-solder metallic core structure 5 to electrically conductive pad 12. Each non-solder metallic core structure 14 may comprise any conductive metallic material that does not comprise solder including, inter alia, copper, gold, nickel, etc or any combination thereof. Each interconnect structure 5 comprises a first non-solder metallic structure 5 a and a second non-solder metallic structure 5 b electrically and mechanically connected to the first non-solder metallic structure 5 a. First non-solder metallic structure 5 a may comprise a first metallic material (e.g., gold) and second non-solder metallic structure 5 b may comprise a second and different (i.e., from the first material) metallic material (e.g., copper). Alternatively, each interconnect structure 5 may comprise only a single non-solder metallic structure (e.g., all copper). Each interconnect structure 5 is thermo-compression bonded (i.e., a bond is formed by using a heating and pressure process in order to form a bond) to an associated electrically conductive pad 10. Each electrically conductive pad 10 may comprise a first material layer 10 a (e.g., comprising a same material a comprised by first non-solder metallic structure 5 a) formed over a second material layer 10 b (e.g., comprising a same material a comprised by second non-solder metallic structure 5 b). Additionally, each second material layer 10 b may comprise a plurality of material layers such as, inter alia, titanium/copper, chromium/copper, titanium/nickel, vanadium/copper, etc. The thermo-compression bond is formed between first material layer 10 a and first non-solder metallic structure 5 a. Each interconnect structure 5 electrically and mechanically connects an electrically conductive pad 10 to an electrically conductive pad 12. Non-solder metallic core structure 5 comprises a cylindrical shape. Solder structure 14 may comprise any solder material suitable for a flip chip interconnections including, inter alia, an alloy of tin such as SnCu, SnAgCu, SnPb, etc. As an alternative, at least a first of non-solder metallic core structures 5 could be replaced by a solder structure such that at least one of electrically conductive pads 10 is connected to an associated electrically conductive pad 12 using a non-solder metallic core structure 5 and at least another of electrically conductive pads 10 is connected to an associated electrically conductive pad 12 using a solder structure.

FIG. 2 depicts a first alternative to FIG. 1 illustrating a cross-sectional view of an electrical structure 2 b, in accordance with embodiments of the present invention. In contrast with electrical structure 2 a of FIG. 1, electrical structure 2 b of FIG. 2 each interconnect structure 5 d is thermo-compression bonded (i.e., a bond is formed by using a heating and pressure process in order to form a bond) to an associated electrically conductive pad 12 instead of soldered. Each interconnect structure 5 d comprises a first non-solder metallic structure 5 a, a second non-solder metallic structure 5 b electrically and mechanically connected to the first non-solder metallic structure 5 a, and a third non-solder metallic structure 5 c electrically and mechanically connected to the second non-solder metallic structure 5 b. First non-solder metallic structure 5 a and third non-solder metallic structure 5 a may comprise a first metallic material (e.g., gold) and second non-solder metallic structure 5 b may comprise a second and different (i.e., from the first material) metallic material (e.g., copper). Alternatively, each interconnect structure 5 d may comprise only a single non-solder metallic structure (e.g., copper). Each electrically conductive pad 12 may comprise a first material layer 12 a (e.g., comprising a same material a comprised by third non-solder metallic structure 5 c) formed over a second material layer 12 b (e.g., comprising a same material a comprised by second non-solder metallic structure 5 b). The thermo-compression bond is formed between first material layer 12 a and third non-solder metallic structure 5 c. Each interconnect structure 5 c electrically and mechanically connects an electrically conductive pad 10 to an electrically conductive pad 12.

FIG. 3 depicts a first alternative to FIG. 2 illustrating a cross-sectional view of an electrical structure 2C, in accordance with embodiments of the present invention. Electrical structure 2C may comprise an optional layer(s) 16 of underfill encapsulant material. In contrast with electrical structure 2B of FIG. 2, electrical structure 2C of FIG. 3 comprises a plurality of spherical interconnect structures 17 comprising a spherical non-solder (i.e., does not comprise any solder material) metallic core structure 17. Each non-solder (i.e., does not comprise any solder material) metallic core structure 17 comprises a non solder metallic layer 17 b (i.e., comprising a first metallic material such as, inter alia, gold) that completely surrounds an exterior surface of an associated non-solder metallic core 17 a (i.e., comprising a second metallic material such as, inter alia, copper). Each non solder metallic layer 17 b may comprise, inter alia, copper, gold, nickel, etc. Each non-solder metallic core 17 a may comprise, inter alia, copper, gold, nickel, etc. Alternatively, each non-solder metallic core structure 17 may comprise a single non-solder metallic material such as, inter alia, copper. Each interconnect structure 17 comprises a first portion 44 a thermo-compression bonded (i.e., comprising a thermo-compression bond 40 a) to electrically conductive pad 10 and a second portion 44 b thermo-compression bonded (i.e., comprising a thermo-compression bond 40 b) to electrically conductive pad 12 thereby electrically and mechanically connecting electrically conductive pad 10 to electrically conductive pad 12. For first level area array interconnects, each non-solder metallic core structure 17 may comprise a diameter of about 25 microns to about 150 microns. For second level area array interconnects (e.g., a ball grid array (BGA)), each non-solder metallic core structure 17 may comprise a diameter of about 0.2 mm to about 1.5 mm. Each non-solder metallic core structure 17 may comprise a core of any conductive metallic material that does not comprise solder including, inter alia, copper, gold, nickel, etc. Additionally, each non-solder metallic core structure 17 may comprise an additional layer(s) of non-solder metallic materials (i.e., different from a material comprised by non-solder metallic core structure 17) surrounding (e.g., see layer 19 in FIG. 3, infra) non-solder metallic core structure 17. The additional layer(s) may comprise any conductive metallic material including, inter alia, nickel, gold, tin, etc. FIG. 4 depicts a first alternative to FIG. 3 illustrating a cross-sectional view of an electrical structure 2D, in accordance with embodiments of the present invention. In contrast with electrical structure 2C of FIG. 3, electrical structure 2D of FIG. 3 comprises solder structures 25. Solder structure 25 comprises solder. Solder is defined herein as a metal alloy comprising a low melting point (i.e., about 100 degrees Celsius to about 340 degrees Celsius) that is used to join metallic surfaces together without melting the metallic surfaces. Each solder structure 25 comprises a portion of solder electrically and mechanically connecting a portion 27 of non-solder metallic core structure 17 to an associated electrically conductive pad 12. As an alternative, at least a first of non-solder metallic core structures 17 could be replaced by a solder structure such that at least one of electrically conductive pads 10 is connected to an associated electrically conductive pad 12 using a non-solder metallic core structure 17 and at least another of electrically conductive pads 10 is connected to an associated electrically conductive pad 12 using a solder structure.

FIG. 5A depicts a first alternative to FIG. 4 illustrating a cross-sectional view of an electrical structure 2E, in accordance with embodiments of the present invention. In contrast with electrical structure 2D of FIG. 4, electrical structure 2E of FIG. 5A comprises a plurality of interconnect structures 21. Each of interconnect structures 21 comprises a non-solder metallic core structure 24 a, a non-solder metallic core structure 24 b, and solder structure 5. Additionally (i.e., optionally), electrical structure 2E comprises an underfill encapsulant layer 16 comprising a first underfill encapsulant layer 16 a and a second underfill encapsulant layer 16 b. Alternatively, underfill encapsulant layer 16 may consist of only a single encapsulant layer. Each non-solder metallic core structure 24 a is thermo-compression bonded to an associated electrically conductive pad 10. Each non-solder metallic core structure 24 a is thermo-compression bonded to an associated non-solder metallic core structure 24 b resulting in an electrical and mechanical connection between each non-solder metallic core structure 24 a to an associated a non-solder metallic core structure 24 b. Each solder structure 5 electrically and mechanically connects a non-solder metallic core structure 24 b to an associated electrically conductive pad 12. The aforementioned connections result in each interconnect structure 21 electrically and mechanically connecting an electrically conductive pad 10 to an associated electrically conductive pad 12. As with each non-solder metallic core structure 17 of FIG. 3, each non-solder metallic core structure 24 a and 24 b of FIG. 5A may comprise a first metallic material such as, inter alia, gold that completely surrounds an exterior surface of a core comprising a second non-solder metallic material such as, inter alia, copper. Additionally, each non-solder metallic core structure 21 a and 21 b may comprise an additional layer(s) of metallic materials surrounding non-solder metallic core structure 24 a and 24 b. Additional layer(s) may comprise any conductive metallic material including, inter alia, nickel, gold, tin, etc. Underfill encapsulant layer 16 a surrounds non-solder metallic core structures 21 a and is in contact with substrate 1. Underfill encapsulant layer 16 b surrounds non-solder metallic core structures 21 b and is in contact with substrate 4. Underfill encapsulant layer 16 a is in contact with underfill encapsulant layer 16 b. Underfill encapsulant layer 16 a may comprise a first material (e.g., a highly filled silica-epoxy composite adhesive) and underfill encapsulant layer 16 b may comprise a second and different material (e.g., a lightly filled silica-epoxy composite adhesive). Underfill encapsulant layer 16 a may comprise a first coefficient of thermal expansion (e.g., comprising a range of about 5-15 ppm/C) that is different (e.g., lower) from a second coefficient of thermal expansion (e.g., comprising a range of about 15-40 ppm/C) comprised by encapsulant layer 16 b. Underfill encapsulent layer 16 a may additionally comprise a filler 16 c dispersed throughout.

FIG. 5B depicts a first alternative to FIG. 5A illustrating a cross-sectional view of an electrical structure 2F, in accordance with embodiments of the present invention. In contrast with electrical structure 2E of FIG. 5A, electrical structure 2F of FIG. 5B comprises interconnect structures 21 a comprising a plurality of solder structures 5 a electrically and mechanically connecting each non-solder metallic core structure 24 a to an associated non-solder metallic core structure 24 b.

FIG. 6A depicts a second alternative to FIG. 5A illustrating a cross-sectional view of an electrical structure 2G, in accordance with embodiments of the present invention. In contrast with electrical structure 2E of FIG. 5A, electrical structure 2G of FIG. 6A comprises interconnect structures 21 b wherein each non-solder metallic core structure 24 a of electrical structure 2G is thermo-compression bonded to an associated electrically conductive pad 12.

FIG. 6B depicts a first alternative to FIG. 6A illustrating a cross-sectional view of an electrical structure 2H, in accordance with embodiments of the present invention. In contrast with electrical structure 2G of FIG. 6A, electrical structure 2H of FIG. 6B comprises interconnect structures 21 c comprising a plurality of solder structures 5 a electrically and mechanically connecting each non-solder metallic core structure 24 a to an associated non-solder metallic core structure 24 b.

FIGS. 7A-7G illustrate a process for generating electrical structure 2C of FIG. 3 and/or electrical structure 2D of FIG. 4, in accordance with embodiments of the present invention.

FIG. 7A illustrates a cross sectional view of substrate 1, in accordance with embodiments of the present invention. Substrate 1 comprises electrically conductive pads 10.

FIG. 7B illustrates a cross sectional view of a transfer substrate 43 comprising a plurality of non-solder metallic core structures 17 a, in accordance with embodiments of the present invention. Non-solder metallic core structures 17 are positioned in cavities 43 a within transfer substrate 43. Each of cavities 43 a comprises similar dimensions as non-solder metallic core structures 17 with cavity positions corresponding to positions of associated electrically conductive pads 10 (i.e., from FIG. 7A). Transfer substrate 43 may comprise, inter alia, glass, silicon, etc. Non-solder metallic core structures 17 may be dispensed into cavities 43 a as a slurry in a solvent such as, inter alia, water alcohol (e.g., isopropanol), etc. The solvent may comprise an appropriate amount of flux (i.e., if generating structure 2D of FIG. 4) to assist in the wetting of solder structures 25 (of FIG. 4) to non-solder metallic core structures 17 a. In a case in which non-solder metallic core structures 17 a are coated with a gold layer 17 b, flux is not necessary. Optionally, the solvent may additionally comprise a small amount of thermally degradable polymeric adhesive to aid in retaining non-solder metallic core structures 17 a in cavities 43 a. Cavities 43 a are fabricated to a size that will only cause one non-solder metallic core structure 17 a to fall into it during a dispensing of non-solder metallic core structures 17 a.

FIG. 7C illustrates a cross sectional view of transfer substrate 43 of FIG. 7D comprising a selected plurality of non-solder metallic core structures 17 a, in accordance with embodiments of the present invention. As an optional feature of the process, transfer substrate 43 may be covered with a polymeric film (i.e., not shown) with through-holes matching a pre-determined fraction of cavities 43 a. The pre-determined fraction of cavities 43 a covered by the polymeric film will be prevented from receiving non-solder metallic core structures 17 a. The pre-determined fraction of cavities 43 a allows a packaging design engineer to selectively place non-solder metallic core structures 17 a. Additionally, solder interconnects or any other type of interconnect (i.e., not shown) may be selectively placed in some of cavities 43 a (i.e., instead of select non-solder metallic core structures 17) for placement on substrate 1. In this option, transfer substrate 43 may be covered with a second polymeric film (i.e., not shown) with through-holes matching the remaining cavities 43 a. The cavities 43 a covered by the polymeric film will be prevented from receiving solder interconnects.

FIG. 7D illustrates a cross sectional view of substrate 1 of FIG. 7A positioned over transfer substrate 43 comprising non-solder metallic core structures 17 a, in accordance with embodiments of the present invention. Substrate 1 of FIG. 7A is positioned over transfer substrate 43 comprising non-solder metallic core structures 17 a in order to transfer non-solder metallic core structures 17 a to substrate 1.

FIG. 7E illustrates a cross sectional view of a structure 23 a comprising substrate 1 after non-solder metallic core structures 17 a have been released from transfer substrate 43 and thermo-compression bonded to electrically conductive pads 10, in accordance with embodiments of the present invention.

FIG. 7F illustrates a cross sectional view of structure 23 a positioned over substrate 4 comprising solder structures 25 of FIG. 4, in accordance with embodiments of the present invention. Structure 23 a is positioned over substrate 4 comprising solder structures 25 in order to form structure 2D of FIG. 4. Structure 2D of FIG. 4 comprises thermo-compression bonds to electrically conductive pads 10 on substrate 1 and solder connections to electrically conductive pads 12 on substrate 4.

FIG. 7G depicts an alternative to FIG. 7F illustrating a cross sectional view of structure 23 a positioned over substrate 4 of FIG. 3, in accordance with embodiments of the present invention. Structure 23 a is positioned over substrate 4 in order to form structure 2C of FIG. 3. Structure 2C of FIG. 3 comprises thermo-compression bonds to electrically conductive pads 10 on substrate 1 and electrically conductive pads 12 on substrate 4.

FIG. 7H depicts an alternative to FIG. 7G illustrating a cross sectional view of structure 23 a positioned over substrate 4 of FIG. 3, in accordance with embodiments of the present invention. In contrast with FIG. 7G, FIG. 7H illustrates optional underfill encapsulant layer 16.

FIGS. 8A-8E illustrate a process for generating electrical structure 2A of FIG. 1 and/or electrical structure 2B of FIG. 2, in accordance with embodiments of the present invention.

FIG. 8A illustrates a cross sectional view of a structure 40 a comprising a sacrificial carrier substrate 35, a release layer 36, and a seed layer 37, in accordance with embodiments of the present invention. Sacrificial carrier substrate 35 may comprise any substrate material including, inter alia, silicon, glass, etc. In order to form structure 40 a, seed layer 37 (e.g., a blanket polymer layer such as, inter alia, a polyimide release layer) is applied to sacrificial carrier substrate 35 and a seed layer 37 (e.g., copper, chromium, etc.) is applied over release layer 36. A photo resist layer (i.e., not shown) may be applied over seed layer 37. The photo resist layer is patterned to form interconnect structures 5 in FIG. 8B. A width for each of interconnect structures selected from a range of about of 10-100 microns with an aspect ratio selected from a range of about 1:1 to 5:1. Interconnect structures 5 may be formed by electroplating of vapor deposition and subsequent chemical/mechanical polishing to insure a flat topography. Alternatively interconnect structures 5 may be formed by a subtractive etch process in which a thick (e.g., 50-100 um) copper layer is applied to seed layer 37 by plating or bonding. The Copper layer would then be coated with photo resist, exposed with an I/O pattern, and subtractive etched down to release layer 36. After removal of the photo resist, interconnect structures 5 may be filled with dielectric or under fill adhesive. A surface of interconnect structures 5 is then planerized in such a way to allow 0.1-1 um of copper protruding above a surface of underfill layer 16 of FIG. 8B. The Copper surface may be bonded directly to electrically conductive pads 10 (see FIG. 8C) or a thin Au or Ni/Au layer 5 a may be added to improve interconnect properties.

FIG. 8B illustrates a cross sectional view of a structure 40 b, in accordance with embodiments of the present invention. Structure 40 b has been formed from structure 40 of FIG. 8A.

FIG. 8C illustrates a cross sectional view of substrate 1, in accordance with embodiments of the present invention. Substrate 1 comprises electrically conductive pads 10.

FIG. 8D illustrates a cross sectional view of structure 40 b of FIG. 8B aligned over substrate 1 of FIG. 8C, in accordance with embodiments of the present invention. Structure 40 b is aligned over substrate 1 so that interconnect structures may be thermo-compression bonded to electrically conductive pads 10 in order to form structure 40C of FIG. 8E. The alignment may be performed using bonding tools by direct viewing through the carrier via infrared or visible light.

FIG. 8E illustrates a cross sectional view of a structure 40C formed after a thermo-compression bonding process and a removal of sacrificial carrier substrate 35 layer a release layer 36 process has been performed, in accordance with embodiments of the present invention. The transfer process comprises heating the aligned assembly (i.e., from FIG. 8D) to a temperature of between about 200 Celsius (C) to about 400 C. The heating process is performed at an inert atmosphere comprising a pressure of 10-100 psi for 5 to 60 minutes. Optional layer of adhesive 22 may be used to enhance a mechanical stability of structure 40C. After the thermo-compression bonding process has been completed, sacrificial carrier substrate 35 may be removed by laser ablation of release layer 36 or by mechanical grinding and etching. Structure 40C is aligned over substrate 4 (i.e., of FIG. 1 or 2) and bonded to substrate 4 in order to form structure 2 a of FIG. 1 or structure 2 b of FIG. 2.

FIGS. 9A-9E illustrate a process for generating electrical structure 2E-2H of FIGS. 5A-6B, in accordance with embodiments of the present invention.

FIG. 9A illustrates a cross sectional view of structure 23 b that is similar to structure 23 a of FIG. 7E after underfill layer 16 a has been formed, in accordance with embodiments of the present invention. Underfill layer 16 a may be applied at wafer-level or on singulated devices. Wafer level underfill may contain a filler 16 c for low coefficient of thermal expansion (CTE). As an alternative, each of non-solder metallic core structures 24 a could comprise associated solder structures 5 a formed over a portion 33 of non-solder metallic core structures 24 a. The aforementioned solder structures 5 a would be used to form structure 2F of FIG. 5B and structure 2H of FIG. 6B.

FIG. 9B illustrates a cross sectional view of a transfer substrate 43 a comprising a plurality of non-solder metallic core structures 24 b, in accordance with embodiments of the present invention. Non-solder metallic core structures 24 b are positioned in cavities 43 b within transfer substrate 43 a. Each of cavities 43 b comprises similar dimensions as non-solder metallic core structures 24 b with cavity positions corresponding to positions of associated non-solder metallic core structures 24 a (i.e., from FIG. 9A). Transfer substrate 43 a may comprise, inter alia, glass, silicon, etc. Non-solder metallic core structures 24 b may be dispensed into cavities 43 b as a slurry in a solvent such as, inter alia, water alcohol (e.g., isopropanol), etc. The solvent may comprise an appropriate amount of flux (i.e., if generating structure 2F of FIG. 5B or FIG. 2H of FIG. 6B) to assist in the wetting of solder structures 5 a (of FIGS. 5B and 6B) to non-solder metallic core structures 24 a. In a case in which non-solder metallic core structures 17 are coated with a gold layer, flux is not necessary. Optionally, the solvent may additionally comprise a small amount of thermally degradable polymeric adhesive to aid in retaining non-solder metallic core structures 24 b in cavities 43 b. Cavities 43 b are fabricated to a size that will only cause one non-solder metallic core structure 24 b to fall into it during a dispensing of non-solder metallic core structures 24 b.

FIG. 9C illustrates a cross sectional view of a structure 23 a of FIG. 11A positioned over transfer substrate 43 a comprising non-solder metallic core structures 24 a, in accordance with embodiments of the present invention. Structure 23 a of FIG. 11A is positioned over transfer substrate 43 a comprising non-solder metallic core structures 24 b in order to transfer and connect non-solder metallic core structures 24 a to non-solder metallic core structures 24 b. Non-solder metallic core structures 24 a are thermo-compression bonded to non-solder metallic core structures 24 b to form structure 2E of FIG. 5A and structure 2G of FIG. 6A. As an alternative, each of non-solder metallic core structures 24 a could comprise associated solder structures 5 a formed over a portion 33 of non-solder metallic core structures 24 a. The aforementioned solder structures 5 a would be used connect non-solder metallic core structures 24 a to non-solder metallic core structures 24 b in order to form structure 2F of FIG. 5B and structure 2H of FIG. 6B.

FIG. 9D illustrates a cross sectional view of structure 23 a of FIG. 11A after non-solder metallic core structures 24 b have been connected to non-solder metallic core structures 24 a, in accordance with embodiments of the present invention. FIG. 9D illustrates a thermo-compression bond between non-solder metallic core structures 24 b and non-solder metallic core structures 24 a. Alternatively, each of non-solder metallic core structures 24 a could comprise associated solder structures 5 a to connect non-solder metallic core structures 24 a to non-solder metallic core structures 24 b.

FIG. 9E depicts an alternative to FIG. 9B illustrating a cross sectional view of a transfer substrate 43 b comprising non-solder metallic core structures 24 a and non-solder metallic core structures 24 b, in accordance with embodiments of the present invention. Non-solder metallic core structures 24 a have been thermo-compression bonded to and non-solder metallic core structures 24 b. Substrate 1 has been positioned over transfer substrate 43 b comprising non-solder metallic core structures 24 a and 24 b so that non-solder metallic core structures 24 a may be thermo-compression bonded to electrically conductive pads 10 with the resulting structure 23 c illustrated in FIG. 9G.

FIG. 9G depicts structure 23 c resulting from the process described with reference to FIG. 9F, in accordance with embodiments of the present invention.

While embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention. 

1. An electrical structure comprising: a first substrate comprising a first electrically conductive pad formed within said first substrate, wherein said first electrically conductive pad comprises a first material layer formed over and in contact with a second material layer, and wherein said first material layer comprises a different material from said second material layer; a second substrate comprising a second electrically conductive pad formed within said second substrate, wherein said second electrically conductive pad comprises a third material layer formed over and in contact with a fourth material layer, and wherein said third material layer comprises a different material from said fourth material layer; and an interconnect structure electrically and mechanically connecting said first electrically conductive pad to said second electrically conductive pad, wherein said interconnect structure comprises a non-solder metallic core structure, wherein a first portion of said non-solder metallic core structure is thermo-compression bonded to said first electrically conductive pad, and wherein a second portion of said non-solder metallic core structure is thermo-compression bonded to said second electrically conductive pad.
 2. The electrical structure of claim 1, wherein said non-solder metallic core structure comprises a spherical shape.
 3. The electrical structure of claim 1, wherein said non-solder metallic core structure comprises a first metallic structure and a second metallic structure covering and in direct mechanical contact with an entire exterior surface of said first metallic structure, wherein said second metallic structure completely surrounds said first metallic structure, wherein said first metallic structure comprises a first metallic material, wherein said second metallic structure comprises a second metallic material that differs from the first metallic material, wherein said first portion of said non-solder metallic core structure is located on said second metallic structure, and wherein said second portion of said non-solder metallic core structure is located on second metallic structure.
 4. The electrical structure of claim 1, wherein said second material layer comprises a first plurality of layers, and wherein each layer of said first plurality of layers are selected from the group consisting of titanium/copper, chromium/copper, titanium/nickel, and vanadium/copper.
 5. The electrical structure of claim 1, wherein said fourth material layer comprises a first plurality of layers, and wherein each layer of said first plurality of layers are selected from the group consisting of titanium/copper, chromium/copper, titanium/nickel, and vanadium/copper.
 6. The electrical structure of claim 3, wherein said first material layer comprises a same material as said second metallic structure.
 7. The electrical structure of claim 3, wherein said third material layer comprises a same material as said second metallic structure.
 8. The electrical structure of claim 3, wherein said first metallic structure comprises gold, and wherein said second metallic structure comprises copper. 